Surface modification of medical implants

ABSTRACT

A method of obtaining a porous titanium surface suitable for medical implants is provided. The titanium surface is exposed to a plasma comprising a reactive plasma gas, the reactive plasma gas comprising an active etching species and a sputtering gas. The plasma conditions are effective to modify the titanium surface and provide surface porosity. The plasma conditions are effective to non-uniformly etch and sputter the titanium surface.

This application is a continuation-in-part application of U.S. Ser. No.08/589,409 filed on Jan. 22, 1996, now U.S. Pat. No. 5,843,289 andsimilarly entitled “Surface Modification of Medical Implants”.

FIELD OF THE INVENTION

This invention relates to a method for modifying surfaces of metallicarticles, such as medical devices. This invention further relates totreatment of implant surfaces to achieve desirable rough and microporoussurface features which enhances bone ingrowth and thus establishes astrong mechanical bond with the implant.

BACKGROUND OF THE INVENTION

Titanium, either pure or as an alloy with a few percent aluminum andvanadium, is used as the metal for bone implantation in knee and hipjoints. Titanium is commonly used because of its mechanical propertiesand also its bio-compatibility. However, titanium does not form a directchemical bond with bone, which can sometimes cause loosening and failureof the implant. Rough and porous implant surfaces permit bone and bonecement to penetrate the surface pores and ideally provide a strongmechanical bond with the implant.

Irregular and rough implant surfaces with certain characteristicfeatures can effectively promote bone ingrowth and directly bond withbone, thus enhancing long term implant stability. European Patent No.038 8576 reports that it is desirable to have a surface macro-roughnesswith pore sizes of about 10-20 μm with a micro roughness superimposedthereon with a pore size of less than 2 μm. Thus, an implant surfacewith macro-roughness and micro-porosity is highly desirable and caneffectively promote bone ingrowth and fixation. This dual featuresurface can also be useful when bone cement material is used to adhereimplants to bone (i.e., polymethyl(methacrylate)).

Achieving the desired balance of macro- and micro-porosity has not beeneasy. Presently, modification of implant surfaces can be classified intotwo distinct groups: (a) surface modification by application of acoating of rough and/or porous material; and (b) surface modification bybulk implant surface treatment.

Porous coatings disclosed in the prior art include diffusion bondedmetal fiber coatings produced from titanium wire in the form of random,rough metal fibers. Pressure sintering of spherical titanium metalpowders or beads on implant surfaces also has been used to producemacro-porosity (U.S. Pat. No. 4,644,942). Thermal plasma spray processeshave also been used to deposit porous coatings (U.S. Pat. No.4,542,539). Commercially pure titanium or titanium alloy powders arepartially melted in the plasma flame and the molten particles impact theimplant surface at high speeds. These powders rapidly quench on thesurface and adhere to the implant metal thus yielding a rough surface.Another porous coating reported in the prior art is a perforated metalfoil applied to a solid metal core (U.S. Pat. No. 3,905,777).

In all of the above prior art devices, an interface between the metalliccore and the porous surface results. In order to be used as a implantdevice, the interface between the coating and the implant substrate mustbe strong and stable. Interfacial failure during implant insertion orafter prolonged use of the implant can result in loose metal particleswhich can become a source of contamination in the adjacent tissue andmay also cause the failure of the implant.

Yet another method of imparting surface roughness to a metallic implantdevice is to treat directly the surface of the solid metallic core. U.S.Pat. No. 4,865,603 discloses a method of surface treatment in which theouter surface is subjected to serial machining processes which resultsin a complex surface topology. A laser beam of selected power and pulseduration has been used to drill cavities on an implant surface. (U.S.Pat. No. 5,246,530). Precise positioning equipment is necessary toprocess the implant in both of these methods. An additional majordisadvantage of the laser process is that it only provides macrocavities for bony ingrowth. Finally, EP 388 576 discloses a method ofaqueous acid treatment, which has been used to achieve a rough andporous surface on implants for bone ingrowth. However, titanium hydrideand other undesirable surface artifacts are formed as a result of theacid-metal reaction.

Thus, there remains a need to provide effective surface modificationtechniques which provide a surface suitable for bone ingrowth, whichpossesses strength and structural integrity and is free of surfacecontamination.

Plasma treatment of titanium-containing surfaces is known and used inthe semiconductor industry, primarily for the etch removal oftitanium-tungsten layers in semiconducting devices. See, U.S. Pat. Nos.5,164,331 to Lin et al. and 4,203,800 to Kitcher et al. The referencesare directed to removal of TiW alloy to expose the underlying layers ina semiconductor device. The material is desirably removed at a constant,even rate across the entire exposed surface. Further, the resultantsurface is desirably smooth and even. These and other similar prior artmethods do not contemplate a process for surface treating a bulktitanium work piece so as merely to alter the surface characteristics orin particular to provide a rough, porous surface.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an article andmethod of manufacture of an article having a porous surface suitable foruse as an implant device.

It is an object of the present invention to provide a porous surface onan implant device with high structural integrity.

It is a further object of the present invention to provide a poroussurface on an implant device with high surface purity and without theformation of surface contaminants.

It is yet a further object of the present invention to provide anarticle and method of manufacturing an article which has a poroussurface exhibiting macro- and micro-roughness.

The present invention uses a process for modifying the implant surfaceto form a porous surface with characteristic macroroughness andmicroporosity, which is close to naturally the surface structure ofoccurring bone tissue. The method of the invention includes exposing atitanium surface to a plasma comprising a reactive plasma gas includingan active etching species and a sputtering ion. Predetermined plasmaconditions are used to modify the titanium surface and provide surfaceporosity.

The present invention also includes exposing a titanium surface to aplasma comprising a reactive plasma gas including an active etchingspecies and a sputtering gas. Predetermined plasma conditions are usedto effect non-uniform etching and non-uniform sputtering of the titaniumsurface. The titanium may be commercially pure titanium or a titaniumalloy.

By “non-uniform etch rate” and “non-uniform sputter rate”, as thoseterms are used herein, it is meant that the etch and/or sputter rate arenot constant or even within a given cross-sectional area of the workpiece surface (geographic non-uniformity). It may additionally includerates which are variable over time (temporal non-uniformity).

By “reactive plasma gas”, as that term is used herein, it is meant toinclude both a gas mixture which is introduced into the plasma chamberand the plasma gases which result therefrom. Thus, the reactive plasmagas includes an active etching species and the halide gases from whichit is formed; the reactive plasma gas likewise refers to a sputteringgas introduced into the plasma and the bombarding ions generatedtherefrom in the plasma.

In a preferred embodiment, plasma conditions are effective to redepositsputtered species onto the titanium surface. In another preferredembodiment, plasma conditions are effective to sputter off oxygenadsorbed on the titanium surface during exposure of the surface to theplasma. In yet another preferred embodiment, a sputtering target isintroduced into the plasma, said sputtering target comprising a maskingelement, such that the masking element is deposited onto the titaniumsurface during exposure of the surface to the reactive plasma gas. Theplasma may be effective to take advantage of masking properties ofalloyed elements in the work piece or of variable etch rates amongsurface deposited species on the work piece.

The present invention provides a rough and/or microporous implantsurface without any contamination or deleterious effect on the bulkproperties of the implant. An advantage of this invention is that therough and porous surface is integral to the implant metal and nochemical byproducts or artifacts are formed by this method. The articlecomprises a metallic substrate; and a surface comprising metallicfilamentous elements having a length of about one to fifteenmicrometers, the filamentous elements integral with the substrate at afirst end and extending in a substantially outward direction from thesubstrate. The article may be characterized by the filamentous elementscharacterized by a fused appearance or by filamentous elementscharacterized by an arched configuration in which the filamentouselements appear fused at a point distal from the substrate.

In one embodiment of the invention, filamentous elements are located atthe substrate with a density of about 1-40 filamentous elements/μm². Inanother embodiment of the invention, the arched filamentous elements arelocated at the substrate with an arch density of about 0.01-10arches/μm². In yet another embodiment of the invention, filamentouselements are located and arranged at the substrate so as to have theappearance of a ridge or wall. The ridge or wall may be separated froman adjacent ridge or wall by a distance of about 0.5 μm to about 2 μm.

By a “porous” as that term is used herein, it is meant a surface whichis not a flat or dense surface and which exhibits a complex surfacemorphology. It does not require the presence of actual “pores” in theconventional sense.

By “filamentous” as that term is used herein it is meant an elongatedfeature extending in a substantially outward direction from thesubstrate surface, having a round, flattened or tape-like appearance.The individual filamentous features may be free standing or they may befused or otherwise agglomerated in the final surface structure. Fused oragglomerated filamentous substructures provide the surface morphology ofthe surface of the inventive devices.

The roughened surface and microporosity on the surface yield an increasein the surface area. Such an increased surface area may also provide anideal substrate for chemically bioactive coatings (for example,hydroxyapatite), thereby improving adhesion of the coating to theimplant surface. Further, the uneven surface morphology, with itsundercuts and deeply etched gaps, are a source of physical interactionwith host site to secure a medical implant and promote osseous ingrowth.

The method of the invention may be readily adapted to non-titaniumsurfaces, by control of the processing variables identified in thepresent invention.

BRIEF DESCRIPTION OF THE DRAWING

For a fuller understanding of the invention, reference is made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic cross-sectional view of the plasma apparatus usedin practice of the invention;

FIG. 2 is a photomicrograph of a titanium surface after treatmentaccording to the invention at 10,000× magnification;

FIG. 3 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 12,000×magnification;

FIG. 4 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 5 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 6 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 7 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 8 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 9 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 10 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 5,000×magnification showing a masked untreated portion of the surface;

FIG. 11 is a scanning electron photomicrograph (SIM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 12 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 25,000×magnification;

FIG. 13 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 25,000×magnification;

FIG. 14 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 15 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 16 is a scanning electron photomicrograph (SEM) of a titaniumsurface after treatment according to the invention at 10,000×magnification;

FIG. 17 is a photomicrograph of a titanium surface before treatment at a5000× magnification; and

FIG. 18 is a graph of illustrating bone contact (%) for various implantsafter six months in vivo implantation in dogs.

DETAILED DESCRIPTION OF THE INVENTION

The present invention uses a reactive plasma etching process to providean article having a roughened, filamentous or porous surface which isparticularly well-suited for bone ingrowth. The implant device ispreferably made of titanium or a titanium alloy, such as alloys withvanadium and/or aluminum; however, as is seen herein below, the processmay be readily adapted to surface treatment of other metal byappropriate selection of reactive plasma species and plasma conditions.

A plasma is a gas (or cloud) of charged and neutral particles exhibitingcollective behavior which is formed by excitation of a source gas orvapor. A reactive plasma generates many chemically active charged(ionic) and neutral (radical) species. These charged and neutral speciesare more active etchants than the original source gas. The reaction ofthe active neutral and ionic species on the surface of a work pieceetches, or removes, portions of the surface. The physical striking ofions on the surface of the implant work piece (i.e., sputtering) furtherenhances the etch rate. Thus in a “reactive plasma” process, bothphysical sputtering of ions and dry chemical etching by the activeneutral and ionic species occurs. Not all of the surface atoms liberatedby sputtering or reactive etch are immediately removed from the systemand, if conditions are favorable, considerable redeposition of the atomsof the titanium surface may occur. Further, the different atoms presenton the exposed surface will undergo reactive chemical etching andphysical sputtering at different rates. Thus, the etch rate andredeposition rate may be controlled by controlling the reactive plasmafeed composition, work piece composition, gas pressure, plasma power,voltage bias across the sheath, reactor geometry, workpiece dimensionand process time.

For application to medical implant devices, it is desirable that themode of etching does not introduce further contaminants onto the surfaceof the work piece. Thus, according to the invention, a fluorine- orchlorine-containing source gas may be used to generate an active etchingchlorine or fluorine species. The active species may be a radical orcharged species. Although iodine-containing gases have not been used toetch titanium, they may be useful to treat other metal surfaces.Titanium reacts with the active etchant species of Cl and F radicals toform a volatile titanium halide which is pumped away by the system. Forexample, TiCl₄ has an evaporation temperature of 135° C.; and TiF₄ has asublimation temperature of 285° C. at atmospheric pressure. Under lowpressure operating conditions, these volatile halides may evaporate atambient temperatures. Thus, no impurities are introduced into the workpiece by the etching process, as all by-products are rapidly removedfrom the work piece surface.

A wide range of fluoro- and chloro-containing compounds may be used togenerate the active chlorine or fluorine species. Suitable compoundsinclude, by way of example only, CF₄, Cl₂, BCl₃, SF₆, CHF₃, CHCl₃, XeF₂,CCl₄, and fluorocarbons commercially available as Freon 11, Freon 12,Freon 13 and Freon 115. Two or more compounds may be used to generate amixture of reactive radicals or to vary the concentration of aparticular reactive radical with changing plasma conditions.

The active etching species is used in combination with a heavierbombarding or sputtering ion and, optionally, lighter species such asoxygen, helium and nitrogen. Bombarding ions of a noble gas and/or alsosome complex reactive ions further enhance the process of the formationand sublimation of the titanium halide at lower temperatures. The ionbombardment assists desorption of metal-halides formed on the surface asthey are knocked off or imparted with enough energy to leave thesurface. This may be contrasted to a wet chemical etching process (e.g.mineral acid bath) in which chemical by-products of the etch remain onand contaminate the work piece surface.

A sputtering gas which can generate heavy bombarding ions is included inthe reactive plasma gases to enhance the reactive etch by physicalsputtering of the work piece surface. Suitable bombarding ions may begenerated from the heavier noble gases, such as Xe, Kr and Ar, orheavier reactive halogen compounds, such as by way of example only,BCl₃, XeF₂, SF₆, etc. The reactive halogen compounds may of course alsobe the source of the reactive chlorine or fluorine species.

Additional lighter gases may be included in the reactive plasma gas,such as helium or nitrogen. These ions affect the ion recombination rateof reactive etching gas. Oxygen may be added to the reactive plasmagases to modify the etch rate of the plasma gas. Oxygen will not effectthe etch rate of fluorine-based plasmas; however, it retards the etchrate of chlorine-based plasmas. Thus, where oxide is formed on thesurface of the work piece surface, etching is hindered.

The reactive plasma gas described above may be used in accordance withthe method of the invention to treat the surface of a work piece andthereby obtain a rough surface having a filamentous substructure. Asillustrated schematically in FIG. 1, a work piece 10 is initiallyultrasonically cleaned with an appropriate solvent (eg., acetone,alcohol) to remove any loose particles from the surface and thoroughlydried. The work piece is then held in a suitable holder 11 and placed ina vacuum chamber 12. The holder 11 and work piece 10 may rest on a baseof the reaction chamber in electrical connection with the non-powerelectrode; or they may positioned in towards the center of the plasmacloud, e.g., suspended by a wire, preferably maintaining electricalconnection with the non-power electrode. It is contemplated that theworkpiece may be rotated or otherwise moved during the etching process.The chamber is evacuated by vacuum pumps (not shown) to about 5×10⁻⁵torr or lower. A chloro- or fluoro-containing gas and a sputtering gas(and other optional gases) are introduced into the chamber by respectiveinlets 13 and 14. The pressure inside the chamber is maintained betweenabout 5 and about 100×10⁻³ torr. A plasma 16 is ignited and createdinside the chamber by a plasma power source 17. The plasma contains highspeed ions and neutral species. These ions bombard the surface of thework piece and cause physical sputtering (removal of atoms fromsurface). Reactive ions and neutral species chemically etch the surface.

The reactive gases are stopped and the chamber is then cleaned with a O₂plasma. The work piece is then removed from the chamber and is cleanedin an ultrasonic bath to remove any surface residue or contamination. Asurface having the morphology such as shown in the photomicrograph(10,000× magnification) in FIG. 2 is obtained. The surface may also bedescribed as having the appearance of coral or cancellous bone, which isrecognized as having desirable properties as a bone substitute material.The surface is characterized by a large surface area to mass ratio,because of the extended open network on the surface. The work piece ispreferably a medical implant device.

A variety of surface morphologies can be produced by the inventivemethod. The array of surface morphologies attainable is best describedas being derived from a filamentous unit substructure. For a given setof etching parameters, a morphology comprising a characteristic patternof filamentous elements or substructures is observed. The filamentouselements or substructures are grouped on the substrate surface in uniqueand characteristic fashions to form “assemblies” which may be lamellar,e.g., corrugated fence- or ridge-like, or which may be tufted, e.g.,coral- or sponge-like.

Filamentous substructures appear clustered and fused with one anotherinto semi-regular morphological assemblies which may be interspersedbetween deeply etched regions or gaps on the article surface. A givensurface morphology may be described in terms of the dimensions of thefilamentous substructure, the degree and nature of filamentous fusionand clustering within the morphological assemblies and the size andspacing of deeply etched gaps. A variety of typical surface morphologieswhich can be produced by the current invention are described below inthese terms and shown in FIGS. 2-16. These morphological descriptionsare descriptive only and in no way are meant to imply how the structuresare actually formed. Specifically, the terms “fused” or “fusion” doesnot imply that initial subunit filaments are formed individually andthen subsequently fused.

Filamentous structure. The filamentous substructure is exemplified inFIG. 3, which is a scanning electron micrograph (SEM) of the surface ofa flat titanium plate prepared by the plasma etching methods of theinstant invention. In this figure, distinct filament-like structures 50are observed approximately 1-3 μm long with a diameter on the order of0.05-0.5 μm. The filaments are present at a density of about 1-4/μm² onthe article surface. Many of the filamentous structures in themicrograph have an appearance of being composed of the fusion of smallerfilamentous subunits (although there is no representation that this ishow they actually are made), and some are ribbon-like in shape, e.g.,see FIG. 14. The degree of fusion may be to the extent that the surfaceappears as a series of ridges with are rather dense, but which retain afilamentous appearance on the ridge walls, see, FIG. 12. The filamentouselements emerge from low ridges 52 (0.1-0.9 μm) on the implant surface.The combination of filamentous substructures on a ridged substratesurface may provide a combination of macro- and micro-surface roughnesswhich has been demonstrated to be desirable for osseointegration ofmedical implants.

Fused appearance. Many of the surface features observed followingetching according to the invention appear as clusters of extensivelyfused filaments. FIG. 4 was taken from the tail of an etched threadedimplant, i.e., a screw. Etching was performed as described in Examples 1and 2 with the sample balanced on its head resting on the non-powerelectrode. The filamentous substructure is clearly visible. In thissample the filaments 60 are at a density of about 2-5/μm² and tend to bemore ribbon-like in appearance. The larger structures 61 appear asribbon-like filaments 60 fused at the base 62 with free ends 64 of about˜1 μm in length projecting from the top. FIG. 5 demonstrates thisfeature where the clustered filaments 70 (2-4 μm in length, 4-10/μm²)are fused throughout their length with only short tips or bumps 72protruding distally from the substrate surface. In FIG. 5 thefilamentous substructures occasionally appear unfused in regionsproximal to the substrate surface near the base but are fused in thedistal regions or at the tips forming holes or arch like structures 76.

Arching. With reference to FIG. 8, the arching morphology is defined asa hole or pore 80 which occurs in a surface assembly which is most oftencharacterized by distally fused filamentous substructures with at leasta portion of the more proximal aspect of the filamentous substructuresbeing distinct or unfused resulting in a hole or pore. Pores may rangeup to 1 μm preferably up to 0.5 μm and most preferably from 0.05 to 0.5μm. Arching may be a predominant morphological feature or it may occurinfrequently. Arches may be open on both sides eg, 82 or closed on oneside to form an alcove-like surface feature, e.g., 84. Arches arebelieved to be an optimal structure for promoting the interaction of thetreated article with living tissue and thus are a preferred surfacefeature on the articles of the invention. FIG. 6 represents a sparselyarched morphology (arch density=0.005-0.06 arches/μm²), whereas FIG. 2is a micrograph of a densely arched, preferred embodiment (archdensity=0.01-10 arches/μm²),

Clustering. The surface features of the article may be further describedas a function of the “clustering patterns” of the fused filamentouselements. Specifically filamentous elements may cluster or be arrangedlinearly to produced a lamellar-like surface morphology 90 such as inFIG. 7. The surface in this figure is characterized by rows 92 of fusedfilaments 94 with interposed deeply etched gaps 96 (0.25-2.0 μm deep).In other cases the clustered filaments 100 may appear leafy andirregular separated by fairly wide (1-4 μm) deeply etched gaps 102 (SeeFIG. 8).

Deeply etched gaps. In some morphologies deeply etched gaps areinterposed between ridges or rows having a clustered filamentousmorphology. Generally these gaps extend down to the base of thefilamentous ridges and represent the full extent of the depth of theetching process. Parameters which favor the production of deep edgedgaps are: long etching time, high RF power, and surface geometry.

Depth. The filamentous surface morphology can be prepared with depths(as measured from an outer edge to-the unetched solid titanium base)from 1 to 15 μm. The various morphological features described above willbe present at various densities depending upon the specific etchingparameters employed.

For the anchoring of biological devices meant to osteointegrate withadjacent bone in the host, the following surface parameters have beenfound to be particularly useful.

Filamentous substructure: filament length: 2-3 μm filament diameter:0.1-0.5 μm filament density: 1-40 μm² filament arch density: .01-10arches/μm² filament clustering: homogenous without dramatic clusteringpatters deeply etched gaps: 1-2 μm without any prominent pattern.

Following etching, the surface may be post processed to further altersurface morphology. Post etching processing treatments include but arenot limited to treatment with acid, heat, abrasion, secondary plasmaetches or coating with particulate, liquids, solids or gels. Aparticularly useful post-etch processing is the coating of the etchedwork piece with a calcium phosphate surface. The calcium phosphate maybe applied in either an amorphous or crystalline structure with one ormore calcium phosphate phases present.

The article of the invention is preferably a medical implant. Medicaldevice implant components which contact bone and for which improvedosseointegration afforded by instant invention may prove advantageousinclude joint replacement components such as knee-tibial, hip-femoral,knee-femoral, hip acetabular, and knee-patellar components. Dental andorthopedic screws such as screws for fixation of trauma devices, screwsfor fixation of orthopedic implants, and pedicle screws may all beadvantageously treated by the current invention. Other orthopedicfixtures include cages for spinal interbody fusions and Kirschner wires.

Medical implant devices with a roughened surface of the invention may besecured at the implant location through the use of orthopedic cementssuch as PMMA or hydroxyapatite bone cements. In a preferred embodimentthe cement is fully resorbable and replaced by the patients own bone.Suitable such cements are described in published PCT applicationPCT/US96/07273 by Lee et al.

A selective combination of the physical and chemical etching processescreates the microporosity. By proper control over the ratio of gasmixtures, gas pressure, plasma power and process time, a surface withinterconnected porosity can be produced. These and other processingparameters are adjusted to predetermined values so as to achievenon-uniform sputtering and non-uniform etch rates of the surface. Etchand sputtering rates are a function of a plurality of parameters, suchas, but not limited to, grains boundaries, impurities, incident ionenergies, neutral species density, and gas pressure. The presentinvention has recognized that it is possible to adjust the etch rate (byvarying incident ion energy, gas pressure, power input, etc) so as toobtain an optimal etch rate for forming porous surfaces. Where etchrates are too high or too low, a porous surface is not formed. Forexample, at low gas pressures, ion energies are high, collisions ofsputtered atoms with neutral species are less frequent. This leads tohigher etch rates (and low redeposition rates). Conversely, at high gaspressures, collisions are more frequent, leading to significantredeposition of the sputtered atoms. Thus the etch rates are slow andthe surfaces are smooth. The optimal etch rate is between these twoextremes.

The etch rate and the surface etch profile rate also are functions ofthe concentration of the active species, i.e., Cl or F, in the plasma.The plasma gas composition, and in particular,. the reactive gas tosputtering gas ratio, and plasma power determine the concentration ofthese active species. The normal operating frequency for reactive ionetch plasma power source is RF power at about 13.56 MHz (a frequencyassigned for industrial use). However lower (100-300 KHz) and higher(microwave −2.45 GHz) frequencies can also be used as plasma powersources. RF plasmas are more efficient at generating plasma at low gaspressures, which give higher sheath voltages and therefore, higher onbombardment energies. High energy bombardment is particularly useful forinsulating surfaces such as metal oxides.

Without being limited to the processes described hereinbelow, the novelspongelike, porous surface structure of the invention (as illustrated inFIG. 2) is believed to result from non-uniform sputtering and etch ratesof the work piece surface. This may be achieved by one or more of thefollowing processes occurring during reactive plasma etching.

(a) Redeposition of sputtered or etched material onto the work piecesurface;

(b) Differing etch rates of titanium and alloyed elements such as Al andV in the work piece resulting in a non-uniform etch rate;

(c) Backsputtering of backing plate atoms (typically Al) causing amasking effect on the work piece surface; and

(d) Selective oxide formation on the surface of the work piece resultingin a masking effect.

By masking, it is meant that the surface of the work piece is covered ormasked and thus is not exposed to the plasma gas. A mask is used in thesemiconductor industry in the formation of gates and other circuitfeatures. The mask is typically a layer deposited on the surface whichis not affected by the surface removal process and thereby protects theunderlying surface. In the present invention, the masking effect isobtained, not by a deliberate deposition of a protective masking layer,but by features of the plasma process. These processing features may beset at predetermined values for the reactive plasma etch in order toobtain the microporosity of the invention.

The present invention identifies the factors which may be controlledduring surface treatment of the titanium work piece. Microporosity isobtained by selective control over composition and gas ratio of thereactive plasma gas, the plasma gas pressure, the plasma power, thecomposition of the work piece and holder. One or more, but notnecessarily all, of the variables may be present and set atpredetermined values in any particular system.

The rate of redeposition is influenced by reactive plasma gas pressure.As the pressure of the system is increased, the density of the variousspecies present increases as well. Species that are ionized and presentin the plasma are more likely to collide with other constituentparticles in the plasma, lose energy and drop back to the surface.Gases, such as helium and nitrogen, may be added to the reactive plasmagas (this is reflected in the total pressure of they system) to increasecollisions and enhance the redeposition rate of the sputtered species.The sheath voltage may also affect the rate of redeposition. High sheathvoltage would increase the rate of removal of Ti from the work piecesurface, by increasing the energy of the bombarding species and thepropensity of the active species (Cl or F) to move to the work piecesurface.

The presence of oxygen on the work piece surface will also contribute tonon-uniform surface modification. In addition, residual oxygen and watervapor in vacuum chamber provides oxygen. Oxygen on the surface typicallyis present in the form of a passivating TiO₂ layer which is notreactively etched by chlorine active species. The passivating layer actsas a mask to prevent reactive chemical etch of the underlying titaniumsurface. Thus, a reactive plasma gas having a chlorine active speciesand a sputtering gas will promote non-uniform surface etching. Sites onthe work piece surface which contain the oxide are not etched by thechlorine active species, while surface sites where oxygen had beenremoved by heavy sputtering gases (and the underlying Ti layer has beenexposed) would react to form titanium halide. Because the sputteringaway of the surface oxygen occurs a little at a time, the underlyingsurface is etched at differing rates.

The use of titanium alloy for the work piece promotes a microporousmicrostructure as well. The non-uniform etch rate is achieved due to theuneven etch rate of the alloyed metals compared to titanium. Forexample, an alloy of Ti, Al and V would have the Al and V etching awayfaster than the Ti, resulting in an uneven, porous structure.

Backsputtering from the work piece holder may also contribute to theformation of the uneven, porous surface. While the incident ions arebombarding the work piece surface, the holder is also being bombarded bythese ions. This causes atoms from the holder to be sputtered(backsputtering). These backsputtered atoms collide with gas species andmay be deposited onto the work piece. If these atoms are not etchedreadily by the active species, they act as a mask while the work piecesurface is etched away around them. Eventually, these masking atoms aresputtered away by the bombarding ions and the surface underneath isexposed to reactive etching. This masking effect at a microlevel resultsin a non-uniform etching rate, promoting the microporosity of theinvention. It is also within the scope of the invention to intentionallyintroduce a masking element from sources other than the holder duringthe plasma etching process. For example, a second target may bepositioned within the chamber for such purposes.

The present invention is thus a non-uniform dry chemical etch andredeposition process without any contamination or effect on the bulkproperties of the implant. The primary advantage of this invention isthat the rough, microporous surface is integral to the implant metal andthat no chemical byproducts or artifacts are formed by this method.

In preferred embodiments, the surface is cleaned before plasma etchingto remove all surface impurities. Conventional cleaning techniques maybe used, such as ultrasonic cleaning and ultra high vacuum-compatiblecleaning methods.

Titanium forms a very thin layer of native oxide (TiO₂) on its surfacewhen exposed to the ambient. Chlorine gas (Cl₂) or the radical speciesCl does not etch this native oxide. Without the removal of this nativeoxide the titanium underneath cannot be etched to create the desiredporous surface. Thus, in embodiments which use an active chlorine plasmaspecies, the process desirably includes a preliminary sputtering step inwhich the passivating TiO₂ layer is removed. In this process, an initialsputtering process is used to remove this extremely thin oxide layer.The sputtering step may be carried out using heavier ions, such as byway of example only, BCl₃, Ar and XeF₂. Plasma etch of titanium oxideswith fluorine will remove the titanium oxide layer and therefore doesnot require the initial oxide “breakthrough” step.

In some embodiments, a macro surface roughness or “macroporosity” may beformed on the work piece surface before the plasma etch surfacetreatment of the invention. Grit or sand blasting of the surface may beused to induce macroporosity. Grit blasting and sand blasting arecommonly used methods for creating a visibly rough surface. The smoothsurface may be exposed to a high velocity stream of sand or grit thatphysically gouges away parts of the surface. This only can producemacroporosity, as the effect is limited by the particle size of theimpacting sand or grit. Macroporosity is typically defined as surfacefeatures on the order of 10 or 20 microns and greater. It may provide asuitable surface, however, on which to perform the microporosityinducing surface treatment of the invention.

In another preferred embodiment, post-etching processes may be performedwhich improve the biocompatibility of a medical implant device. Forexample, the porous surface of the treated titanium work piece may becoated with a hydroxyapatite coating to enhance bioactivity at thesurface. The hydroxyapatite coating may be in the range of about 100Angstroms to about 1 μm in thickness. The thickness is chosen tomaintain the microporosity. A suitable method of coating may be RFsputtering or ion beam sputtering of hydroxyapatite onto the surface.The interested reader is directed to U.S. Pat. No. 5,543,019 to Lee etal. for further details, which is herein incorporated by reference.

A medical implant having the surface described herein is ideally suitedfor in-growth of natural bone. Superior osseal in-growth into boneimplants has been established in dogs. Titanium screws treated accordingto the invention were implanted in a jaw tooth socket and the bonein-growth was monitored over time by determining the % surface areacontact of the screw with ingrown bone. FIG. 4 is a bar graphillustrating the percent bone contact of surface treated implants aftersix months in vivo. A comparison implant having a surface roughnesscharacterized by spikes demonstrated only 38% bone contact, while theimplant having the interconnecting porous surface of the inventiondemonstrated 55% bone contact and 54% bone contact, with and without anHA coating, respectively. See, Example 4 for details.

It will be readily apparent that the present invention may be extendedto articles which are comprised of metals other than titanium. In orderto obtain a porous surface for non-titanium articles, an active specieswhich is capable of reactive etching that metal is selected and used incombination with a sputtering gas. Plasma conditions are selected toobtain an etch rate which is optimal for forming a porous surface.

It is contemplated that the surface of the present invention may finduses in areas other than the bone implant field. For example, the highsurface area/low mass ratio which is possible for such as surface may beuseful for applications requiring surface/incident radiationinteraction, such as thermal and electron radiation. They may beparticularly useful as transducers.

The invention may be understood with reference to the following exampleswhich are presented for the purpose of illustration only and which arenot intended to be limiting of the invention, the true nature and scopeof which are set forth in the claims hereinbelow.

Example 1. This is an example to demonstrate modifying a planarcommercially pure (CP) Ti surface.

FIG. 17 is a photomicrograph of the CP titanium surface beforetreatment, showing the smooth features of the work piece. Some machiningmarks are evident on the surface, but it is otherwise smooth andnonporous. The implant was thoroughly cleaned before it is loaded in thevacuum chamber. The implant was rinsed in distilled water,ultrasonically cleaned in trichloroethane and rinsed with a successionof toluene, acetone and ethanol; however, conventional high vacuumcompatible cleaning methods can be employed.

After cleaning, the implant was placed on the electrode holder insidethe chamber. The chamber was closed and pumped down to show a vacuum ofabout 5×10⁻⁵ torr. The chamber was then cleaned with an oxygen plasma(O₂ pressure, 200 millitorr; RF power, 150 W) for 5 minutes to removeany traces of residue or contamination.

The process of reactive ion etch was carried out in two stages, (a) anoxide “breakthrough” step and a (b) reactive ion etching step. Areactive mixture of Cl₂ and BCl₃ was used along with the noble gas He asthe reaction gases.

(a) Oxide breakthrough—A mixture of BCl₃ (20 sccm), Cl₂ (10 sccm) and He(30 sccm) was introduced in the chamber and the pressure inside thechamber is maintained at 50 millitorr (sccm=standard centimeter cube perminute). The RF power source was operated in the voltage mode. The poweris turned on and the sheath voltage across the plasma and the implantwork piece is maintained at 300 V. The power reads 200 Watts. Theprocess time is only 30 seconds.

(b) Reactive Etch—The reactive mixture ratio then was altered to BCl₃(15 sccm) and Cl₂ (20 sccm) and He remained same at 30 sccm. Thepressure was reduced to 40 millitorr. The RF power source is nowoperated in the power mode. The power is turned on and maintained at 100W. The sheath voltage reads 250 V. The process time is 1 hr.

After etching was completed, the power was turned off and the chamberwas evacuated of its reactive gases to a pressure of 5×10⁻⁵ torr. O₂ wasthen introduced inside the chamber (30 sccm) the pressure inside thechamber is set at 200 millitorr. The RF power is turned on to 150 W andthe cleaning process time is 15 minutes. This removes any residualhydrocarbon contamination on the surface of the implant work piece. Thetitanium work piece was then taken out and ultrasonically cleaned.

FIG. 2 is a photomicrograph of the titanium surface after the foregoingtreatment. The surface is porous and uneven after surface treatment andspongelike or coral-like in appearance. The micropores are about 1.0 to2 microns in diameter and about 2 to 3 micron deep. Such a rough andmicroporous surface can then be coated with extremely thin or thick HAcoating to enhance bioactivity at the surface.

Example 2. This is an example to demonstrate modifying a Ti alloysurface. The work piece is made of

A work piece made of titanium alloyed with vanadium and aluminum(Ti-6Al-4V) is thoroughly cleaned before it is loaded in the vacuumchamber. The implant is rinsed in distilled water, ultrasonicallycleaned in trichloroethane and rinsed with a succession of toluene,acetone and ethanol.

After cleaning, the implant is placed on the electrode holder inside thechamber. The chamber is cleaned with an oxygen plasma (O₂ pressure, 200millitorr; RF power, 150 W) to remove any traces of residue orcontamination. The process of reactive ion etch may be carried out intwo stages, (a) an oxide “breakthrough” step and a (b) reactive ionetching step, as described in Example 1. A reactive mixture of Cl₂ andBCl₃ may be used along with the noble gas He as the reaction gases. Thealloyed aluminum and vanadium in the titanium work piece promote thenon-uniform etch rate of the surface to obtain a microporous surfacewith interconnection porosity.

Example 3. This example illustrates the preparation of an hydroxyapatitecoated work piece.

A work piece is surface treated as described in either Example 1 or 2.The microporous surface is then coated with a thin hydroxyapatitecoating as described in U.S. Pat. No. 5,543,019 to Lee et al. Theresultant article has a thin hydroxyapatite coating which issufficiently thin so as not to diminish the porosity of the titaniumsurface.

Example 4. This example illustrates the osseal in-growth into thesurface of the invention.

The purpose of this study was to determine the rate at which the testsurfaces of a medical implant is fully integrated into living bonetissue in canines, which were chosen because historically they havehistorically been used for dental and orthopedic research. Twenty-fouranimals were used—four groups of three male dogs and four groups ofthree female dogs at one year old.

Phase I. Bilateral extractions of the mandibular premolars (PM1-PM4) andthe first molar (M1) were performed. The procedure was performed underfull anesthesia and aseptic surgical conditions. Prior to extractions,maxillary and mandibular impressions were made with the aid of customtrays and polyvinylsiloxane heavy bodied and light bodied material(Express, #M,Minneapolis-St. Paul).

The animals were anesthetized with Acepromazine (1.0 mg/kg i.m.) andsodium penabarbital (25 mg/kg i.m.). The animal's vital signs were takenbefore and throughout the procedure. The animal was then tested forproper anesthetic depth by applying pressure to the pad of the foot andobserving for a response. After obtaining adequate anesthesia, a fullthickness mucoperiosteal flap is raised adjacent to the mandibularpremolars with the use of a #15 scapula blade and a periosteal elevator.The teeth were then hemisected through their furcation using a smallfissure burr in a high speed hand piece with irrigation. Extractions ofthe separated PM1-PM4 and M1 teeth were performed using elevators andforceps. Guided Tissue Augmentation Membranes (GTAM) were subsequentlyplaced over the extraction sites followed by flap closure with singleinterrupted 4-0 Gortex® sutures (W. L. Gore, Flagstaff, Ariz.).Bicilline (600,000 u, i.m.) was administered every 48 hours up to sixdays and Ibuprofen (400 mg/day) up to four days post surgery to each dogfor infection and pain control. During recovery, each dog was monitoredin a recovery cage until awakening. In order to achieve maximum alveolarbone healing, a three month post extraction healing period wasnecessary. The animals were kept on a soft diet due to teeth lossresulting from this procedure.

Phase II. In each of the study groups of three dogs, one dog was given acomparison rough surface titanium screw implant, one dog was given atitanium screw implant having the porous surface of the invention andone dog was given a titanium screw implant having a thin HA coating overthe porous surface of the invention. The surface of 8 mm long, 3.3 mmdiameter Hollow Screw Implants (Institute Straumann, AG, WaldenburgSwitzerland) were subjected to the titanium plasma surface treatment ofthe invention. Calcium phosphate coatings were subsequently applied tohalf of these implants.

After three months post extraction, surgical implantation was performed.Preparation of the implant bed involved a traumatic surgical techniquesunder sterile conditions. The anesthesia protocol is that described inPhase I. After obtaining adequate anesthesia l thickness mucoperiostealflaps we aised bilaterally over the healed mandibular extraction sites,followed by removal of the GTAM. During implant bed preparation,drilling procedures were carried out at rotations of 400 to 600 rpm andunder continuous cooling using an H₂O. Five implant sites were prepared8 mm apart in the mandible bilaterally. Penetration of the cortical bonesurface of the healed alveolar ridge was accomplished with round burs ofdiameter 1.4 a nd 2.3 mm. The implant beds were prepared to a depth of 8mm by means of 2.2 and 2.8 mm drills. Eight mm implants were chosen inorder to achieve primary stability without encroaching upon the inferioralveolar nerve and vessels. Twist drills with a diameter of 3.3 weresubsequently used to obtain the final bed dimensions.

Phase III. X-rays were taken monthly throughout the study. At the end ofsix months, the animals were sacrificed and the tests sites were removedand examined by histology. FIG. 18 is a graph illustrating the extent ofbone contact with the implant after six months. The screws having theinterconnecting porosity of the present invention demonstratedsignificantly greater bone in-growth than the comparison roughenedsurface. There was little or no difference in screws with or without thehydroxyapatite coating.

Example 5. This example illustrates use of a medical implant devicetreated as described in Example 1. A bone filler paste is preparedaccording to copending application Ser. No. 08/729,344. Bilateral toothextractions and rough surface titanium screw implants are preparedaccording to Example 4. Following extraction the tooth socket is filledwith the paste and the implant is inserted into the paste prior to pastehardening. The subject is allowed to heal for three months at which timeprosthetic teeth are attached to the embedded screw.

Examples 6-12. A series of titanium implant devices were subjected to aplasma etch according to the procedure set forth in Example 1 andprocess using the conditions described in Table 1, below. Screws weremounted either head down, with the tail extending upward-into the plasmacloud or were mounted horizontally within the plasma cloud. The surfacemorphology of the result and devices are show in FIGS. 3-12. Theseexperiments demonstrate that a wide variety of surface features may beobtained using the method of the invention to provide articles withnovel and desirable surface features.

TABLE 1 Surface treatment parameters for Examples 6-12. R.F. SurfaceExample Duration BCl₃ Cl₂ He/O₂ power Voltage pressure sample featuresNo. Process step (min) (sccm) (sccm) (sccm) (W) (V) (mTorr) locationshown in: 6 breakthrough  0:30 20 15 30 300 50 vertical FIG. 9; etch75:00 15 20 30 100 300 40 FIG. 11 clean 15:00 0 0 30 150 300 200 7breakthrough  0:45 20 15 30 300 60 vertical FIG. 8 etch 60:00 15 25 30100 300 80 clean 15:00 0 0 30 150 300 200 8 breakthrough  0:30 20 15 30300 50 vertical FIG. 16 etch 60:00 10 25 30 100 300 50 clean 15:00 0 030 150 300 200 9 breakthrough  0:30 20 15 30 225 300 50 vertical FIG. 13etch 75:00 15 25 30 100 175 40 FIG. 15 clean 15:00 0 0 30 150 75 200 10breakthrough  0:30 20 15 30 250 300 50 vertical FIG. 5; etch 75:00 10 2530 300 40 FIG. 10 clean 15:00 0 0 30 150 300 200 FIG. 14 11 breakthrough 0:30 20 15 30 200 300 50 vertical FIG. 4 etch 60:00 10 25 30 125 275 40clean 15:00 0 0 30 150 150 200 12 breakthrough  0:30 20 15 30 180 300 50horizontal FIG. 7 etch 75:00 15 25 30 75 170 40 in the clean 15:00 0 030 150 175 200 cloud

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexample be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. An article, comprising: a metallic substratehaving a surface comprising metallic filamentous elements having alength of about one to fifteen micrometers, each filamentous elementhaving a first end integral with the substrate and extending in asubstantially outward direction from the substrate, wherein at least twofilamentous elements have spaced apart first ends and merge at alocation a distance from the substrate.
 2. The article of claim 1,wherein the merged filamentous elements form arches on the surface ofthe substrate.
 3. The article of claim 1, , or , wherein the metallicsurface comprises titanium.
 4. The article of claim 1, , or , whereinthe article is a medical implant.
 5. The article of claim 1, , or ,wherein the filamentous elements on the substrate have a density ofabout 1-40 filamentous elements/μm².
 6. The article of claim 2, whereinthe arched filamentous elements on the substrate have an arch density ofabout 0.01-10 arches/μm².
 7. An article, comprising: a metallicsubstrate having a surface comprising metallic filamentous elements,each filamentous element having a first end integral with the substrateand extending in a substantially outward direction from the substrate,wherein at least one plurality of filamentous elements collectively forma pattern, each plurality of filamentous elements collectively defininga wall or a ridge on the surface of the substrate.
 8. The article ofclaim 7, wherein at least one wall or ridge is separated from anadjacent wall or ridge by a distance of about 0.5 μm to about 2 μm. 9.The article of claim 3, wherein the metallic substrate comprises atitanium alloy.
 10. The article of claim 9, wherein the titanium alloycomprises an element selected from the group consisting of aluminum andvanadium.
 11. The article of claim , wherein the irregular morphologyforms a pattern defining tufts of filamentous elements on the surface ofthe substrate.
 12. An article, comprising: a metallic substrate having asurface comprising metallic filamentous elements having a length ofabout one to fifteen micrometers, each filamentous element having afirst end integral with the substrate and extending in a substantiallyoutward direction from the substrate, wherein at least one plurality offilamentous elements are joined at first ends and expand outwardly fromthe joined ends.
 13. The article of claim , wherein a plurality of theoutwardly extending filamentous elements collectively provides amorphology resembling a member of the group consisting of fingers,tufts, sponges and fans.
 14. The article of claim 1, 12, or , whereinthe filamentous element length is in the range of about 2 to 3 microns.15. The article of claim 7 or , wherein adjacent filamentous elementsare fused along the entire length thereof.
 16. The article of claim 1,7, 12 or , wherein adjacent filamentous elements are fused over aportion of the length thereof.
 17. The article of claim 1, wherein themerged filamentous elements form pores with the substrate, the poreshaving a pore size of up to about 1 micron.
 18. The article of claim 1,wherein the merged filamentous elements form pores with the substrate,the pores having a pore size in the range of about 0.05-0.5 microns. 19.The article of claim 1, wherein the merged filamentous elements form analcove with the surface of the substrate.
 20. The article of claim 7,wherein the wall or ridge further includes free ends of the filamentouselements projecting above the top of the wall or ridge.
 21. An article,comprising: a metallic substrate having a surface comprising metallicfilamentous elements, each filamentous element having a first endintegral with the substrate and extending in a substantially outwarddirection from the substrate, wherein a plurality of filamentouselements are positioned and arranged to collectively define undercutswith respect to the surface of the substrate.
 22. The article of claim1, wherein the merged filamentous elements are arranged to collectivelydefine undercuts with respect to the surface of the substrate.
 23. Thearticle of claim 1, wherein the merged filamentous elements are arrangedto form undercuts on the surface of the substrate.
 24. The article ofclaim 1, wherein pores are formed between the merged filamentouselements, the pores having a pore size of up to about 1 micron.
 25. Thearticle of claim 1, wherein pores are formed between the mergedfilamentous elements, the pores having a pore size in the range of about0.05-0.5 microns.